1. Field of the Invention
The present invention relates to a process for the preparation of (meth)acrylic polymers or copolymers of at least one (moth)acrylic monomer by ultra-fast anionic polymerization leading to a polymer having controlled mass and polydispersity and a high solids content.
2. Description of Prior Art
The production of polymers with a high solids content has many advantages, in particular economic advantages (high concentration of polymer at the end of polymerization, reduced amount of solvent making the final polymer recovery easier).
The anionic polymerization of (meth)acrylic monomers is, in general, a fast polymerization, so much that it is more efficient to carry it out continuously. However, it is sometimes so fast that it is difficult to control.
To solve the problem of controlling the polymerization in order to obtain a "monodisperse" polymer, that is to say one with an Mw/Mn ratio, or polydispersity, in the region of 1, Japanese patent application JP-A-60-56910 proposes to use a static mixer which makes it possible to mix the flow of monomer and the flow of initiator quickly and uniformly before the polymerization in a tubular reactor.
The intended aim of obtaining a polydispersity in the region of 1 is achieved. However, there are numerous drawbacks.
The appropriate concentration of the monomer(s) in the monomer flow is theoretically from 1 to 50% by weight. However, the viscosity of the solution of the polymer formed increases with the concentration, thus making the mixing operation difficult and consequently leading to an even wider molecular mass distribution of the final living polymer. The only two examples of this Japanese patent application use monomer solutions of low concentration, leading, after polymerization, to a medium having a low solids content (less than 10% by weight assuming a conversion of 100%).
Moreover, it is recommended in this Japanese patent application, in order to avoid side reactions, to maintain a polymerization temperature of -100.degree. C. to +20.degree. C. and, in particular, in the case of polar monomers such as (meth)acrylates, to perform the polymerization at a temperature which is preferably below -40.degree. C. The polymerizations described in the examples are carried out at -78.degree. C.
In addition, the polymerization reactor is preferably tubular, in order to facilitate the exchange of heat due to the exothermy of the reaction. This type of reactor is not, however, suitable for ultra-fast polymerization reactions since the polymerization time is too short to be able to exchange the heat due to the exothermic reaction of the polymerization.
It is known that the kinetics of anionic polymerization depend on the polymerization conditions, particularly on the reaction temperature and the concentration of the initiator system.
The teaching of the Japanese patent application mentioned above leads to the use of an initiator system and polymerization conditions which bring about a propagation constant which, at -40.degree. C. (preferred upper temperature limit for the case of the methyl methacrylate monomer, according to this document), would be about 100 l mol.sup.-1 s.sup.-1, that does not permit ultrafast reactions.
On the other hand, with an initiator system such as that described in document EP-A-524,054, that is to say a mixture of an initiator and a ligand of the alkoxyalkoxide type, the propagation constant for the reaction (kp(t)) is, for methyl methacrylate, greater than or equal to 10.sup.4 l mol.sup.-1 s.sup.-1 at 0.degree. and 10.sup.3 l mol.sup.-1 l s.sup.-1 at -40.degree. C. With such an initiator, the reaction is thus ultra-fast, to such an extent that it is difficult to control even when it is performed continuously. (The propagation constants mentioned above, both for the case of the Japanese patent application and for the case of the document EP-A-524,054, were determined, by the Applicant, by the method of G. V. Schultz in European Polymer Journal--Vol. 10, pages 121-130--1974).